Referring to FIG. 6, one type of a typical multiple scan line barcode reader 100, such as those used in many fixed scanners, is described. The multiple scan lines are generated by the use of a spinning polygon mirror 110. The polygon mirror 110 has a plurality of facet mirrors 111, 112, 113, 114 arranged at different tilt angles forming the sides of the polygon. A reading scan beam 160 generated from a scanning beam source 150, such as a laser diode for example, passes through an aperture in a mirror 154 and is directed onto the polygon mirror 110 by a collection lens 156 while the polygon mirror rotates. The mirror 154 gathers the return scan beam reflected from a barcode symbol and focuses it onto a photo sensor 158 for the purpose of decoding the barcode symbol.
As the polygon mirror 110 rotates, the scan beam 160 is reflected by each facet mirror onto one or more of down stream pattern mirrors 170, . . . 176 that are arranged around the polygon mirror 110. Each of the down stream pattern mirrors project the scan beam into a different area of the scan volume creating a scan line. By providing the multiple pattern mirrors 170, . . . 176 positioned at various angles about the rotating polygon mirror 110 a desired number of scan lines can be projected into the scan volume. The polygon mirror is typically driven by a brushless DC (BLDC) motor.
In some modern barcode reading applications, such multi-scan line barcode readers are provided as a modular dual-mode hands-free unit. Such dual-mode hands-free scanners operate as a multi-scan line fixed scanner as well as a single scan line hand-held barcode scanner. In the fixed scanner mode, the scanner is positioned in a designated place scanning the scan volume with the multiple scan lines. In the hand-held mode, if necessary, the scanner can be configured to project only one of the multiple scan lines into the scan volume allowing the user to use it as a traditional single scan line hand-held scanner.
In conventional dual-mode scanners, the single scan line mode is implemented by incorporating a device, external to the BLDC motor driving the polygon mirror, to detect the angular position of the polygon mirror and turning the reading scan beam on and off synchronized with the polygon mirror so that only the scan beam that is reflected by a desired mirror facet is projected into the scan volume. This produces a single scan line.
In many cases the external device used to detect the position of the polygon mirror is an external Hall Effect sensor. However, the use of such external sensor device also requires mounting a magnet on the polygon mirror. The use of such extra hardware increases the manufacturing cost of such dual-mode barcode scanners.
Therefore, a further improvement is desired in dual-mode barcode scanners.